As shown in, for example, Japanese Patent Laid-Open No. 2008-138569, a technique of controlling an air-fuel ratio of an internal combustion engine using the output of an exhaust gas sensor has been conventionally used. An exhaust gas sensor issues an output corresponding to the air-fuel ratio of an exhaust gas by reaching an activation temperature. There is a demand for early start of air-fuel ratio control using an exhaust gas sensor output in order to obtain a favorable emission characteristic early at the time of start of an internal combustion engine. In order to satisfy the demand, a heater is usually provided inside an exhaust gas sensor, and the heater quickly heats the exhaust gas sensor to a predetermined activation temperature at the time of start of an internal combustion engine.
Incidentally, there is the case in which the output from an exhaust gas sensor can be used without any problem if only specific conditions are satisfied, even before the exhaust gas sensor reaches the activation temperature, that is, even before the regular activation state. Thus, even before the regular activation state, by determining the state in which the output of the exhaust gas sensor is usable, that is, a semi-activation state, the output of the exhaust gas sensor can be used for air-fuel ratio control at an earlier stage.
However, while an internal combustion engine stops, exhaust gas components adsorb on the electrode portions of the exhaust gas sensor, the porous body portion of the sensor element and the like. Hereinafter, the exhaust gas components which adsorb on the exhaust gas sensor will also be generically described as “adsorbed species”. While the exhaust gas sensor is heated at the time of start of an internal combustion engine, the adsorbed species start to be desorbed. The adsorbed species which are desorbed are present in the vicinity of the exhaust gas sensor, and thereby, influences the output of the exhaust gas sensor, and inhibit accurate measurement of the exhaust gas air-fuel ratio. While the effect of the adsorbed species remains, the exhaust gas sensor output does not indicate the accurate value of the air-fuel ratio of the exhaust gas. As above, there is the problem that an output shift occurs due to the effect of the adsorbed species which are desorbed, and early use of the exhaust sensor output is inhibited.
Thus, in the air-fuel ratio control device according to Japanese Patent Laid-Open No. 2008-138569, the output value of the exhaust gas sensor is masked during the time period until the exhaust gas sensor reaches a regular activation state after a semi-activation state under the situation in which the effect of the adsorbed species is worried about. Thereby, the measures for preventing the adverse effect of the output shift due to adsorbed species can be taken in accordance with necessity.